Humanity's Grassroots: How Grazing Animals Shaped Evolution

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Secrets regarding the spread of the world's grasslands — which
proved vital to countless species of grazing animals and may have
influenced humanity's evolution — have now been uncovered in
fossil teeth, scientists reveal.

These new findings show when the ancestors of elephants, rhinos,
hippos, cattle, pigs and other lineages of herbivores began
grazing on grasses, helping create the landscape wherein our own
species developed.

Grasslands now cover more than 20 percent of the planet's lands,
but the savannahs of Africa only emerged shortly before the
hominids that gave rise to humanity evolved. Although the
cool-season, or so-called C3, grasses developed more than 65
million years ago,
soon before the end of the age of dinosaurs, the warm-season,
or C4, grasses found in savannahs evolved 25 million to 35
million years ago and emerged in Africa just 10 million years or
so ago.

"Corn is C4, sugarcane is C4," researcher Kevin Uno, a geologist
at the University of Utah, told LiveScience. "Although C4 grasses
make up less than 1 percent of all plant species in the world,
they are responsible for 30 percent of all biomass on Earth.
Humans evolved in a world where C4 grasses were available. These
are the plants we rely on for food. They made a big impact."

To understand how animals in Africa adapted to these new grasses,
switching over from a diet of leaves from C3 trees and shrubs to
C4 grasses, Uno and his colleagues investigated 452 fossil teeth
of herbivores from three sites in Kenya also occupied by
ape-like human ancestors.

"The opening of the landscape, which we can see in the increasing
abundance of grazing mammals, led eventually to the conditions
where the lineage of primates, leading to us, were able to
evolve," researcher Thure Cerling, a geologist at the University
of Utah, told LiveScience. "Without the opening of the landscape
and development of savannas, we would not have evolved."

In the kind of photosynthesis that C3 plants rely on, the
carbon-12 isotope, which has the lightest molecular weight of
stable carbon isotopes, is preferred. In C4 photosynthesis, both
carbon-12 and the heavier carbon-13 isotope are used. Depending
on the food eaten, an animal would integrate certain ratios of
carbon isotopes into its body tissues. By measuring ratios of
these isotopes in the enamel of fossil teeth and determining the
ages of the sediments they were found in, the scientists could
figure out if herbivores ate C3 plants, C4 plants or a mix, and
when they "hit the hay," so to speak, constructing a
7-million-year record of dietary change from 10 million to 3
million years ago.

"Grass is now the main food for many herbivores in East Africa,"
Uno said.

The first animals to switch to warm-season grasses were zebras'
ancestors, starting 9.9 million years ago. "Horses were ready for
the 'new restaurant' and went exclusively to this new food
resource," Cerling said.

Next, some but not all rhinos made the switch beginning 9.6
million years ago. This was also true of the bovids, which today
include gazelles, wildebeest
and Cape buffalo. Grass-grazing spread 7.4 million years ago to
the ancestors of elephants, and once it did, they remained
grazers until very recently, probably in the last million years
or so. Today, African and Asian elephants eat mostly C3 trees and
shrubs.

Hippos began grazing on grass more slowly, as did suids, the
ancestors of bush pigs and warthogs. And giraffes never left the
salad bar of trees and shrubs, in part because their long necks
are designed to get to leaves up high, not grass down low.

The first herbivores to eat these C4 plants had longer teeth —
meant for tearing through leaves — that took more time for the
abrasive grasses to wear down. The increased availability of
these grasses meant there was a new food source available for any
herbivore to try if they could digest them, as they have more
cellulose and lower nutritional quality than most C3 plants.

"If you lived in a town that only ate beef for dinner and the
frozen fish stick guy came through selling TV dinners at half
price because everyone ate beef, wouldn't you at least try fish
for dinner?" Uno asks. "That example is a bit anthropomorphized,
but I see C4 grasses as new resources that may not have been as
much in demand as C3 cool-season grasses, trees and shrubs. This
could be because some animals had a hard time digesting C4
grasses."

It remains a mystery as to why the C4 plants became as important
in global ecosystems as they did, Cerling said. No evidence of
widespread grasslands has been found earlier than 4.2 million
years ago in East Africa.

"Only during the past 1 million years did grasslands become as
dominant as they are today in East Africa," Uno said. Still, even
before 4.2 million years ago, "there was enough C4 grass around
for a whole bunch of animals to make a living off of it," he
added.

Global or regional changes in climate have the potential to
change a forest into grassland or vice versa. When this happens
over large areas, animals must alter their diets or deal with the
consequences, which in extreme cases could mean moving to a new
habitat or eventually going extinct.

"As we continue to add to atmospheric carbon dioxide through the
burning of fossil fuels, we'll give C3 plants a competitive
advantage," Uno said. "We don't know the full series of potential
outcomes of that, but it's certainly an issue that plant
ecologists are well-attuned to."

The scientists detailed their findings online today (April 4) in
the journal Proceedings of the National Academy of Sciences.